1. Field of the Invention
This invention relates to the field of optical devices that manipulate optical energy of tightly controlled optical wavelength, particularly for use in communication applications. More particularly, the invention relates to lasers which produce optical energy of a specified wavelength and which can be tuned or switched to other specified wavelengths by thermal means.
2. Description of the Related Art
Over the past several years, there has been ever-increased interest in tunable lasers for use in optical communication systems, in general, and for use in dense wavelength division multiplexing (DWDM) applications, in particular. DWDM allows high bandwidth use of existing optical fibers, but requires components that have a broad tunable range and a high spectral selectivity. Such components include tunable lasers that should be able to access a large number of wavelengths within the S-band (1490-1525 nanometers), the C-band (1528-1563 nanometers), and the L-band (1570-1605 nanometers), each different wavelength separated from adjacent wavelengths by a frequency separation of 100 MHz, 50 MHz, or perhaps even 25 MHz.
The distributed Bragg reflector (DBR) laser was the first such tunable laser used in optical communication. The DBR laser consisted of a semiconductor amplifier medium, defining an active section, and an optical waveguide. The optical waveguide included a portion without a grating that defined a phase control section and a portion in which a single grating of typically constant pitch (xcex9) was formed which constituted a distributed Bragg reflector or, more simply, the Bragg section that reflected light at the Bragg wavelength xcexB. Wavelength tuning of such a DBR laser was performed by transferring heat into the phase control section, the Bragg section, or both. The optical waveguide was defined by an organic layer which constituted a core with another organic confinement layer disposed both above and below the core. Wavelength tuning of such a DBR laser was performed by either injecting current or transferring heat into the phase control section, the Bragg section, or both. Injecting minority carriers made it possible to vary the refractive index of the waveguide and thus control the Bragg wavelength xcexB by the equation xcexB=2neff xcex9 where xcex9 is the pitch of the grating and neff is the effective refractive index of the waveguide. Alternatively, a pair of heating resistance strips was disposed on opposite outer surfaces of the laser component for the phase control section, the Bragg section, or both. By independently controlling the voltages to the heating resistance strips, the temperature and hence the index of refraction of the organic layers that form the optical waveguide was controlled via the thermo-optical effect. Tuning by injecting current had the disadvantage of increasing optical loss and adding optical noise. Tuning by heating had the disadvantage of increasing optical loss and adding optical noise. Both options induce long-term drift in the Bragg wavelength thereby reducing reliability. For a more detailed discussion of a wavelength tunable DBR laser by heating, please refer to U.S. Pat. No. 5,732,102 by Bouadma entitled xe2x80x9cLaser Component Having A Bragg Reflector of Organic Material, And Method of Marking Itxe2x80x9d which is hereby incorporated by reference.
A super structure grating distributed Bragg reflector (SSG-DBR) laser was another type of tunable laser that held great promise. The InGaAsP3-InP SSG-DBR laser was comprised of a semiconductor amplifier medium with an InGaAsP/InGaAsP multiple quantum wells active region, an SSG-DBR section on both sides of the semiconductor amplifier medium, and a phase control section between one of the SSG-DBR sections and the semiconductor amplifier medium. Thin film Pt heaters were formed on the top surface and corresponding electrodes were formed on the bottom surface of each SSG-DBR section and the phase control section. The two SSG-DBR sections were used as mirrors with different sampling periods giving different peak separations and different reflective combs in the reflectivity-wavelength spectrum. In the reflectivity-wavelength spectrum, only one reflective peak associated with each SSG-DBR section coincided and where these reflective peaks coincided at a cavity mode, that cavity mode was selected for lasing. Wavelength tuning of the SSG-DBR laser was performed by injection current into or heating of either SSG-DBR section or the phase control section. Current injection into or heating of the SSG-DBR sections changed the refractive index of each waveguide and shifted the reflection spectrum of each SSG-DBR section. Similarly, current injection into or heating the phase control section shifted the cavity modes. While providing a broad tuning range, wavelength tuning by injection current caused considerable spectrum line width broadening and a decrease in emitted power, both important criteria in DWDM applications. Further, the long term affects of wavelength tuning by injection currents on SSG-DBR laser performance remains unknown. In addition, current SSG-DBR lasers are monolithic devices fabricated from InGaAsP/InP and the manufacture of such SSG-DBR lasers results in low yield because of the immaturity of the InP or GaAs based processing technology. For a more detailed discussion of a wavelength tunable SSG-DBR laser by injection current, please refer to a paper by Ishii et al. entitled xe2x80x9cNarrow Spectral Linewidth Under Wavelength Tuning in Thermally Tunable Super-Structure-Grating (SSG) DBR Lasers,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2, Pages 401-407, June 1995, which is hereby incorporated by reference.
For a more detailed discussion of the state of the art on widely tunable lasers, please refer to a paper by Rigole et al. entitled xe2x80x9cState-of-the-art: Widely Tunable Lasers,xe2x80x9d SPIE, Vol. 3001, Pages 382-393, 1997, which is hereby incorporated by reference.
Embodiments of novel tunable lasers are disclosed which can quickly and repeatedly access a broad range of relevant wavelengths with high spectral selectivity yet without the problems associated with the prior art.
A first embodiment of the novel tunable laser includes a substrate fabricated of a first material that supports a gain means, a first waveguide, and a second waveguide. The gain means is fabricated of a second material and includes an active emission layer that generates optical energy. The active emission layer includes a first and a second facet. The first waveguide includes a first core and a first end on the first core, which may include a first taper, is adjacent to the first facet to receive the optical energy. The first core is fabricated from an inorganic material and the first waveguide is fabricated from both inorganic and thermo-optical organic material. A first reflector receives the optical energy propagating along the first waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of first reflection wavelengths. The second waveguide includes a second core and a first end on the second core, which may include a taper, is adjacent to the second facet and receives optical energy. The second core is fabricated from an inorganic material and the second waveguide is fabricated from both inorganic and thermo-optical organic material. A second reflector receives the optical energy propagating along the second waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of second wavelengths. Between the first end of the first reflector and the first reflector along a reflector free-portion of the first waveguide, there may be a phase control section which can slightly shift the Fabry-Perot resonant cavity modes associated with the tunable laser. Thermo-optical organic material is disposed to shift the plurality of first reflection wavelengths, the plurality of second reflection wavelengths, and the Fabry-Perot resonant cavity modes in response to changes in the temperature in the thermo-optical organic material. Tuning of the laser may be achieved by changing the temperature in the thermo-optical organic material which has an index of refraction that varies in response to changes in temperature. By varying the temperature of heaters or coolers in the thermo-optical organic material associated with the first reflector, the second reflector, the phase control portion, or combinations thereof, a broad wavelength tuning range with high spectral selectivity is possible.
A second embodiment of the novel tunable laser includes a substrate fabricated of a first material that supports a gain means and a waveguide. The gain means is fabricated of a second material and includes an active emission section, which generates optical energy, and includes a facet. The waveguide includes a core and an end on the core, which may include a taper, is adjacent to the facet to receive optical energy. The core is fabricated from an inorganic material and the waveguide is fabricated from both inorganic and thermo-optical organic material. A first reflector receives the optical energy propagating along the waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of first reflection wavelengths. A second reflector receives the optical energy propagating along the waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of second wavelengths. Between the end and the first reflector and between the first and second reflectors, both along a reflector free-portion of the waveguide, there may be phase control sections which can slightly shift the Fabry-Perot resonant cavity modes associated with the tunable laser and an etalon formed by the first and the second reflectors. Thermo-optical organic material is disposed to shift the plurality of first reflection wavelengths, the plurality of second reflection wavelengths, and the Fabry-Perot resonant cavity modes in response to changes in the temperature of the thermo-optical organic material. Tuning of the laser may be achieved by changing the temperature in the thermo-optical organic material which has an index of refraction that varies in response to changes in temperature. By varying the temperature of heaters or coolers in the thermo-optical organic material associated with the first reflector, the second reflector, the phase control portions, or combinations thereof, a broad wavelength tuning range with high spectral selectivity is possible.
A third embodiment of the novel tunable laser includes a substrate that supports a gain means and a waveguide. The gain means includes an active emission layer, which generates optical energy, and includes a facet. The waveguide includes a core and an end on the core, which may include a taper, is adjacent to the facet and receives the optical energy. The core is fabricated from inorganic material and the waveguide is fabricated from both inorganic and thermo-optical organic material. A reflector receives the optical energy propagating along the waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of first reflection wavelengths. Thermo-optical organic material is disposed to shift the plurality of reflection wavelengths in response to changes in the temperature in the thermo-optical organic material. Tuning of the laser may be achieved by changing the temperature in the thermo-optical organic material which has an index of refraction that varies in response to changes in temperature. By varying the temperature of heaters or coolers in the thermo-optical organic material, a broad wavelength tuning range with high spectral selectivity is possible.
The thermo-optical organic material of the tunable laser is preferably selected so as to have a high coefficient of variation in refractive index as a function of temperature, the magnitude of which should be preferably greater than 1xc3x9710xe2x88x924/xc2x0 C. Examples of thermo-optical organic material used in the tunable laser and that exhibit these characteristics include polymers derived from methacrylate, siloxane, carbonate, styrene, cyclic olefin, or norbornene.
An integrated optical component is also disclosed for the second embodiment of the tunable laser above. The integrated optical component includes all the functional elements associated with the respective embodiment of the tunable laser, but does not include the gain means that is typically fabricated from a different material than the integrated optical component.
It should be observed that, except for the gain means, the tunable laser is fabricated using Si processing technology and only the gain means is of GaAs, InP, InGaAsP, or other exotic semiconductor materials which requires complex and sensitive processing technology, such as epitaxial growth and cleaving. The gain means is independently fabricated with a minimum of structure. Accordingly, the tunable laser is easy to manufacture, cost effective to manufacture, and results in high yield.